A domestic electric toaster typically comprises a toasting chamber into which slices of bread or other toastable foodstuffs are introduced, a source of radiant heat in the form of a plurality of electric heating elements and a cabinet housing these components. A carriage is provided in order to assist the user in inserting and removing the bread. Conventionally, domestic toasters have slots in their upper surface into which bread may be inserted. The carriage is then lowered vertically, the heating elements are energised and the bread is toasted. At the end of the toasting operation, the carriage is raised so that the toast protrudes upwardly from the slots for removal by the user.
The duration of the activation of the heating elements controls the degree of browning of the toast. Domestic toasters usually include a manually adjustable dial to enable the user to select the degree of browning, in response to which the toaster controls the duration of the activation of the heating elements. In many domestic electric toasters, the activation of the heating elements is controlled solely on a time basis, with the duration of the toasting operation decreasing with the required degree of browning. However, any one setting of the dial can produce different degrees of browning depending, inter alia, on the nature of the foodstuff, its moisture content and the ambient temperature at the start of the toasting operation, and so alternative techniques for controlling the duration of the activation of the heating elements have been explored.
One such technique is described in GB 2,199,733, in which the foodstuff to be toasted is illuminated using a light source. A light detector is located alongside the light source to produce a signal whose magnitude is proportional to the amount of reflected light received by the detector. A circuit is provided to terminate the toasting operation when the signal output from the detector indicates that the amount of reflected light has fallen to a selected percentage of its initial value, with the selected percentage being chosen depending on the required degree of browning, for example 50% for lightly toasted bread, and 20% for very well done bread. While the initial value of the reflected signal would be different for white and brown bread, the relationship between the degree of browning and the percentage reduction in the signal from the detector was found to be substantially the same for both bread types.
A problem associated with this technique for detecting the degree of browning of the toasted foodstuff is that the intensity of the illumination of the toasting chamber will tend to increase during a toasting operation. When hot, the heating element will glow red. However, when the heating element is relatively cold at the start of a toasting operation, there is an initial period of time, for example between 10 and 30 seconds depending on the nature of the heating element and voltage of the power supply, during which the heating element does not glow. Consequently, the initial value of the reflected signal will tend to indicate that the foodstuff to be toasted is darker than it actually is, which can result in the foodstuff being toasted to a different degree than required by the user.